1. Field of the Invention
The present invention relates to an information storage medium such as a so-called erasable optical disk having a recording later irradiated with a beam to cause changes in optical characteristics, thereby writing information therein or erasing it therefrom.
2. Description of the Prior Art
Some conventional erasable optical disks subjected to information erasure in addition to information read/write access utilize transformation between crystalline and amorphous phases. In a conventional optical disk of this type, the recording later is locally exposed to a beam to cause a transformation between the crystalline and amorphous phases at the applied site and to read information by utilizing different complex indices of refraction of the phases. For this reason, the optical disk is made of a material easily transformed into an amorphous phase upon beam radiation and subsequent rapid cooling. Such materials are chalcogenide-based materials such as TeO.sub.x, Te-Sn-Se, and Ge-Te (see "Erasable Large-Capacity Optical Disk", 116th research meeting, 131st committee, the Japan Society for the Promotion of Science (May, 1983), for TeO.sub.x).
The amorphous phase of chalcogenide-based materials, however, is not stable, and stable information recording cannot be performed. Moreover, if a material having a high crystallization temperature is used in order to stabilize the amorphous phase, it takes a long period of time to cause a transformation from the amorphous phase to the crystalline phase in the information erase mode.